POLYPLOIDY: TYPES, IN ANIMALS, IN HUMANS, IN PLANTS - BIOLOGY - 2025

The polyploidy is a type of genetic mutation is the addition of a full complement (complete sets) of chromosomes to the cell nucleus, forming homologous pairs. This type of chromosomal mutation is the most common of the euploidias and is characterized by the fact that the body carries three or more complete sets of chromosomes.

An organism (usually diploid = 2n) is considered polyploid when it acquires one or more complete sets of chromosomes. Unlike point mutations, chromosomal inversions, and duplications, this process is large-scale, that is, it occurs on complete sets of chromosomes.

Source: Haploid_vs_diploid.svg: Ehambergderivative work: Ehamberg

Instead of being haploid (n) or diploid (2n), a polyploid organism can be tetraploid (4n), octoploid (8n), or greater. This mutation process is quite common in plants and is rare in animals. This mechanism can increase genetic variability in sessile organisms that are not able to move around.

Polyploidy is of great importance in evolutionary terms in certain biological groups, where it constitutes a frequent mechanism for the generation of new species since chromosomal load is an inheritable condition.

When does polyploidy occur?

Chromosome number disturbances can occur both in nature and in laboratory-established populations. They can also be induced with mutagenic agents such as colchicine. Despite the incredible precision of meiosis, chromosomal aberrations do occur and are more common than one might think.

Polyploidy arises as a result of some alterations that can occur during meiosis either in the first meiotic division or during prophase, in which homologous chromosomes are organized in pairs to form tetrads and a nondisjunction of the latter occurs during the anaphase I.

Appearance of new species

Polyploidy is important as it is a starting point to originate new species. This phenomenon is an important source of genetic variation, since it gives rise to hundreds or thousands of duplicate loci that are left free to obtain new functions.

In plants it is particularly important and quite widespread. It is estimated that more than 50% of flowering plants have originated from polyploidy.

In most cases, polyploids differ physiologically from the original species and due to this, they can colonize environments with new characteristics. Many important species in agriculture (including wheat), are polyploids of hybrid origin.

Types of polyploidy

Polyploidies can be classified according to the number of complete chromosome sets present in the cell nucleus.

In this sense, an organism that contains "three" sets of chromosomes is "triploid", "tetraploid" if it contains 4 sets of chromosomes, pentaploid (5 sets), hexaploidae (6 sets), heptaploid (seven sets), octoploid (eight games), nonaploidae (nine games), decaploid (10 games), and so on.

On the other hand, polyploidies can also be classified according to the origin of the chromosomal endowments. In this order of ideas, an organism can be: autopolyploid or allopolyploid.

An autopolyploid contains several sets of homologous chromosomes derived from the same individual or from an individual belonging to the same species. In this case, polyploids are formed by the union of non-reduced gametes of genetically compatible organisms that are cataloged as the same species.

An allopolyploid is that organism that contains non-homologous sets of chromosomes due to hybridization between different species. In this case, polyploidy occurs after hybridization between two related species.

Polyploidy in animals

Polyploidy is rare or infrequent in animals. The most widespread hypothesis that explains the low frequency of polyploid species in higher animals is that their complex mechanisms of sex determination depend on a very delicate balance in the number of sex chromosomes and autosomes.

This idea has been maintained despite the accumulating evidence of animals that exist as polyploids. It is generally observed in lower animal groups such as worms and a wide variety of flatworms, where individuals usually have both male and female gonads, facilitating self-fertilization.

Species with the latter condition are called self-compatible hermaphrodites. On the other hand, it can also occur in other groups whose females can give offspring without fertilization, through a process called parthenogenesis (which does not imply a normal meiotic sexual cycle)

During parthenogenesis, offspring are basically produced by mitotic division of parental cells. This includes many species of invertebrates such as beetles, isopods, moths, shrimp, various groups of arachnids, and some species of fish, amphibians, and reptiles.

Unlike plants, speciation through polyploidy is an exceptional event in animals.

Examples in animals

The Tympanoctomys barriere rodent is a tetraploid species that has 102 chromosomes per somatic cell. It also has a "gigantic" effect on your sperm. This allopolyploid species probably originated from the occurrence of several hybridization events of other rodent species such as Octomys mimax and Pipanacoctomys aureus.

Polyploidy in humans

Polyploidy is uncommon in vertebrates and is considered irrelevant in the diversification of groups such as mammals (as opposed to plants) due to disruptions that occur in the sex determination system and in the dose compensation mechanism.

An estimated five out of every 1000 humans are born with serious genetic defects attributable to chromosomal abnormalities. Even more embryos with chromosomal defects miscarry, and many more never make it to birth.

Chromosomal polyploidies are considered lethal in humans. However, in somatic cells such as hepatocytes, about 50% of these are normally polyploid (tetraploid or octaploid).

The most frequently detected polyploidies in our species are complete triploidies and tetraploidies, as well as diploid / triploid (2n / 3n) and diploid / tetraploid (2n / 4n) mixoploids.

In the latter, a population of normal diploid cells (2n) coexists with another that has 3 or more haploid multiples of chromosomes, for example: triploid (3n) or tetraploid (4n).

Triploidies and tetraplodia in humans are not viable in the long term. Death at birth or even a few days after birth has been reported in most cases, varying from less than one month to a maximum of 26 months.

Polyploidy in plants

The existence of more than one genome in the same nucleus has played an important role in the origin and evolution of plants, being perhaps the most important cytogenetic alteration in plant speciation and evolution. Plants were the gateway to the knowledge of cells with more than two sets of chromosomes per cell.

From the beginning of chromosomal counts, it was observed that a great variety of wild and cultivated plants (including some of the most important) are polyploid. Almost half of the known species of angiosperms (flowering plants) are polyploid, likewise most ferns (95%) and a wide variety of mosses.

The presence of polyploidy in gymnosperm plants is rare and highly variable in groups of angiosperms. In general, it has been pointed out that polyploid plants are highly adaptable, being able to occupy habitats that their diploid ancestors could not. Furthermore, polyploid plants with more genomic copies accumulate greater “variability”.

Within plants, perhaps the allopolyploids (most common in nature) played a fundamental role in the speciation and adaptive radiation of many groups.

Horticultural improvement

In plants, polyploidy can originate from several different phenomena, perhaps the most frequent being errors during the meiosis process that give rise to diploid gametes.

More than 40% of the cultivated plants are polyploid, among them alfalfa, cotton, potatoes, coffee, strawberries, wheat, among others, without a relationship between domestication and polyploidy of the plants.

Since colchicine was implemented as an agent to induce polyploidy, it has been used in crop plants for basically three reasons:

-To generate polyploidy in certain important species, as an attempt to obtain better plants, since in polyploids there is usually a phenotype in which there is a notable growth of “gigabytes” due to the fact that there are a greater number of cells. This has allowed notable advances in horticulture and in the field of plant genetic improvement.

-For polyploidization of hybrids and that they regain fertility in such a way that some species is redesigned or synthesized.

-And finally, as a way to transfer genes between species with different degrees of ploidy or within the same species.

Examples in plants

Within plants, a natural polyploid of great importance and particularly interesting is bread wheat, Triticum aestibum (hexaploid). Along with rye, a polyploid called “Triticale” was intentionally built, an allopolyploid with the high productivity of wheat and the robustness of rye, which has great potential.

Wheat within cultivated plants has been remarkably essential. There are 14 species of wheat that have evolved by allopolyploidy, and they form three groups, one of 14, another of 28 and a last of 42 chromosomes. The first group includes the oldest species of the genus T. monococcum and T. boeoticum.

The second group is made up of 7 species and apparently derives from the hybridization of T. boeoticum with a species of wild grass from another genus called Aegilops. The crossing produces a vigorous sterile hybrid that through chromosome duplication can result in a fertile allotetraploid.

The third group of 42 chromosomes is where the bread wheat is located, which probably originated through hybridization of a tertraploid species with another species of Aegilops followed by a duplication of the chromosomal complement.

References

1. Alcántar, JP (2014). Polyploidy and its evolutionary importance. Issues deficiency and Technology, 18: 17-29.
2. Ballesta, FJ (2017). Some bioethical considerations in relation to the existence of cases of human beings with complete tetraploidy or triploidy, born alive. Studia Bioethica, 10 (10): 67-75.
3. Castro, S., & Loureiro, J. (2014). The role of reproduction in the origin and evolution of polyploid plants. Ecosistemas Magazine, 23 (3), 67-77.
4. Freeman, S and Herron, JC (2002). Evolutionary Analysis. Pearson Education.
5. Hichins, CFI (2010). Genetic and geographic origin of the tetraploid rodent Tympanoctomys barriere (Octodontidae), based on the analysis of mitochondrial cytochrome b sequences (Doctoral dissertation, Institute of Ecology).
6. Hickman, C. P, Roberts, LS, Keen, SL, Larson, A., I´Anson, H. & Eisenhour, DJ (2008). Integrated Principles of Zoology. New York: McGraw-Hill. 14 ^th Edition.
7. Pimentel Benítez, H., Lantigua Curz, A., & Quiñones Maza, O. (1999). Diploid-tetraploid myxoloidy: first report in our setting. Cuban Journal of Pediatrics, 71 (3), 168-173.
8. Schifino-Wittmann, MT (2004). Polyploidy and its impact on the origin and evolution of wild and cultivated plants. Brazilian magazine of agrociencia, 10 (2): 151-157.
9. Suzuki, DT; Griffiths, AJF; Miller, J. H & Lewontin, RC (1992). Introduction to Genetic Analysis. McGraw-Hill Interamericana. 4 ^th Edition.